With this article we will understand why Audison Virtuoso 3.0 MPC (Mobile Phase Correction) technology is so important to achieve the highest listening quality in the car cabin. Sound waves have amplitude and phase characteristics. Amplitude is how powerful the wave is. The higher the pressure peaks, the louder the sound is perceived to be.
Phase describes the timing of the wave - when the pressure peaks are occurring. Humans can often hear if the phase of the sound wave is inverted 180 degrees (we can’t tell which one is better, as a rule, but we can often hear the change!) So, if there is only one speaker driver, phase isn’t really crucial. For various reasons, we usually listen to music on speaker systems with multiple driver elements, not a single speaker driver. Once you have more than one speaker driver, their outputs can reinforce each other or conflict with each other.
A key part of designing good-sounding speaker systems is ensuring the different drivers all “play nicely” with each other, without interference.
When two waves of identical frequency interact - when two identical sounds collide - the result can be louder than either sound on its own (constructive reinforcement), or the result can be quieter than either sound on its own (destructive interference), or the result can be identical in loudness to the previous sounds (which is also destructive interference, actually).
Two waves, aligned (+6dB)
Two waves, slightly misaligned (less than +6dB)
Two waves, “out of phase” - one-half wavelength of misalignment, 180 degrees, which results in nearly complete cancellation (-30dB)
Which of these is the case depends on the phase of the two sounds - if they were aligned when they collided, or not - and if not, how far off they were misaligned from each other. This means that whenever we have two identical sounds arriving at different times, we could end up with a louder result, or a quieter result, or no change at all. In most cases, when we use more speaker drivers we are looking for more output, and anything else is a problem. When can we have multiple sounds arriving at the same point at different times?
One cause is when the sounds are played at the same time by different speakers. The output of these speakers might be phase-shifted by the crossover filters used on the signals, or it could be phase shifted due to the combination of the slow speed of sound, different speaker locations, and the resulting varied path-length distances the sound must travel to the listener.
The next three causes have to do with path-length differences:
Two flat signals, summed together, after 69cm of path-length difference
These phase misalignments lead to ragged and uneven frequency responses - often with deep cancellations which are clearly audible and unpleasant. Signals can be phase-shifted by the processing we apply to them. Analog crossover filters and EQ filters always have side effects on the signal. IIR digital filters - the most common type of digital filters - mimic analog filters, including these side effects, so we can inadvertently cause some phase problems as we try to solve other problems.
Finally, once the signal is sent to a speaker, and the speaker does its job and converts it into sound, all sorts of things happen to those sound waves. Often a sound arrives at the listening position directly from the speaker, and then arrived again, a split-second later, after reflecting off of a windshield or trim panel. If these two sounds are out of phase due to the different paths they traveled to get to the listener, cancellation is the result. Reflections are the least significant source of cancellations, and we can’t change the locations and shape of the interior once the installation is complete. OK, all that is a long-winded way of saying that both the phase and the amplitude get all messed up in cars - partially because of where we sit, and partially because of the hard reflective surfaces, partly because the signal processing we use has its own phase side effects, etc., etc. So, if we want to EQ the signal to sound better, we need to do something about cancellations first. If two speakers are fighting each other, we can’t EQ out the resulting dip in the response - so we need to eliminate those conflicts before we have a signal that will even respond well to equalization.
The most common way to address cancellations is to use delay - which eliminates the cancellations caused by path-length differences, but only at one listening position. It doesn’t address any other causes of phase shifts and cancellations - just path-length differences.
So when we tune autosound systems, we have usually applied delay - which corrected a lot of these cancellations - then equalized the acoustic result using IIR EQ and crossover filters (even though our IIR filters would damage the phase even more), and then we called it a day. But now, we have Mobile Phase Correction, powered by Percept, in the flagship Audison bit Virtuoso.
First off, many of the Virtuoso’s internal processes are performed using FIR filters. FIR filters are more difficult to calculate and use than the much-more common IIR filters - most DSP chipsets can’t use FIR - but they can be defined so they don’t affect the phase at all, so they are phase linear on the outputs. All IIR filters mimic the analog capacitor-and-coil filters we used to use to build our own passive crossover networks back in the day. Any IIR filters have phase side effects, but FIR filters don’t have to. And that’s how these FIR filters have been since the early days of the bit One HD. The acoustic signal was eventually going to be damaged in the acoustic realm, but at least we weren’t making it worse in the electrical domain! This was a rare thing - every car-audio DSP other than the bit One HD uses IIR filters. That’s what most of us know.
Afterwards, the bit Virtuoso uses a combination of delay, and a comprehensive FIR filter, to optimize the acoustic result - to get the amplitude and the phase to be linear. This uber-filter affects both the amplitude AND the phase response at the listening position! Unlike IIR equalization, MPC is performed using an FIR filter based on the measured impulse response at the listening position, so the result is phase linear at the listening position!
Now, when you use the bit Tune with 3.0 software, and a bit Virtuoso with 3.0 software, and you use the “Auto EQ and TA” feature, the Virtuoso uses a combination of delay and FIR phase adjustment to automatically create an acoustic response that’s as close as we can get to sitting in the sweet spot in a home listening environment. This sort of FIR digital correction is only available in implementations designed for home systems, but the Percept algorithm used in Virtuoso has been intended for car audio from its initial conception. It’s the only one of its kind!
The “Auto EQ and TA” function is automated - the FIR filter is calculated and applied, and then you can make adjustments to the frequency response in several ways, including the MPC response window that gives us direct control of the FIR filter’s frequency response! If you want to learn more please watch Audison Virtuoso 3.0 webinar below:
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